A crack sensor is proposed that detects a crack (fissure, crackle) generated in a construction such as a bridge, a tunnel and the like, for example, a two-dimensionally extending crack which extends horizontally after extending vertically (see Japanese Unexamined Patent Application Publication No. Hei 11-211644).
In the two-dimensional crack sensor, a first planar resistor is arranged on the surface of an insulator base adhered on the wall surface of an inspected object. An intermediate insulator is arranged on the surface of the first planar resistor. A second planar resistor is arranged on the surface of the insulator. The first and the second planar resistors are separately formed into a structure consisting of a plurality of parallel resistance wires which is deployed at intervals. Moreover, the resistance wires of the first planar resistor are orthogonal to those of the second planar resistor.
The first and the second planar resistors are connected with the lead wire connecting sections arranged at two ends of each resistance wire. For example, the resistance value of the first planar resistor changes when a resistance wire of the first planar resistor is cut off. The extension of a crack is determined by measuring the current between the lead wire connecting sections located at two ends of each resistance wire.
In the two-dimensional crack sensor disclosed in Japanese Unexamined Patent Application Publication No. Hei 11-211644, through a laminated structure in which the intermediate insulator is sandwiched between the first planar resistor and the second planar resistor, the outmost insulator base is adhered on the surface of a measured object such as concrete.
As the crack generated on the concrete extends, the insulator base is broken with the extension of the crack. Consequentially, the resistance wires of the first planar resistor are broken.
However, as the intermediate insulator is sandwiched between the second planar resistor and the insulator base, it is hard for the breaking force resulting from the breaking of the insulator base to spread to the resistance wires of the second planar resistor, and the resistance wires of the second planar resistor may not be broken. In this case, the effect of the two-dimensional sensor cannot be exerted fully.